My Raspberry Pi 2B failed to boot this beta image; stuck part way through. Took me a while to track down the problem: config.txt file in the image specifies arm_freg 1200 - 2B; Pi2B will not run at that speed. Commented out those arm_freq lines, now running.
For me it troubleshooting was complicated as I used new SD card, and suspected that, first time with Etcher, suspected that, image has multiple partitions, that was confusing, and I usually set up SD to enable SSH with ssh file in boot, so did not have HDMI console to see what was going on. And I believed you when you said image was back compatible Not sure why you added the arm_freq, as I think those are the defaults for Pi3B anyway?